1. Field of the Invention
The invention relates to electronic admittance matching networks, and more particularly, to an admittance matching network utilizing a saturable inductor and adapted for matching a radio frequency power source to a variable impedance load such as a gas plasma.
2. Description of the Prior Art
Matching networks are used, particularly in radio frequency applications, for matching the impedance or admittance of a power source to a load having a different impedance or admittance in order to provide maximum power transfer to the load and to preclude damage to the power source from reflected energy due to the mismatch. Such applications include, for example, matching radio frequency amplifiers to an antenna and semiconductor processing apparatus, such as used for sputter deposition of thin films and gas plasma or reactive ion etching to etch conductive patterns on semiconductor wafers.
Apparatus for deposition and etching of semiconductors are widely used in industry. Efficient operation at the relatively high power levels utilized requires that the signal source be admittance matched to the load to be processed. Conventional gas plasma chambers, for example, present a variable admittance to the power source, depending on the processing parameters and the stage of processing. Since efficient power transfer requires providing at least an approximate match between the power source and the load, prior art systems have introduced a matching network having variable impedance parameters which readjusted during the process to maintain an impedance or admittance match.
Prior art matching networks have relied on mechanically varying a plurality of capacitors or inductors, which suffers from slow response and lack of mechanical reliability. One such system utilizes a saturable reactor in solenoidal form as the variable element. However, the solenoid is an inefficient element for this application, as it is an open magnetic circuit with high flux losses. Another application suggests to utilize a toroidal configuration, but toroidal windings are not feasible for the large diameter transmission lines required at the usual power levels, and the close coupling of the windings permits high voltage breakdown and transfer of rf energy to the inductor current supply.
Other prior art deals with methods of sensing and adjusting the variable inductor in response to changing load conditions, but may require complex and expensive circuitry.
The present invention provides a simple, cost effective structure for admittance matching, permitting dynamic control under varying load conditions typical of semiconductor processing apparatus, and wholly electronic in nature. The invention will be described with respect to the accompanying drawings, in which like reference numerals refer to like or similar elements.